Electrical power has always been a limiting factor for satellites, and it restricts the services that they can perform. The need for additional transponders to satisfy the demand for satellite-supplied television, e-mail, worldwide web, long distance telephones, rapid computer data transfer, and many other types of telecommunication is increasing. The number of transponders has risen from about 24 active transponders per satellite in the late 1980's to 94 active transponders on the latest Hughes satellite launched in late 1997. The demand for additional power for transponders over the last few years fits an exponential curve and the end is not in sight. The only practical limitation for generation of the additional power for transponders is the availability of electricity from the solar panels carried on the satellite. At present the size of satellite solar panels is effectively at a maximum. Additional satellites in the same "space slot" can be deployed to increase the total solar panel area, and this is the direction that many satellite companies are going. A major drawback of this approach is that the output signals of the various satellites are not in phase, so interference between satellite transmissions can be a problem. A bigger drawback, however, is that the multiple satellite approach is very expensive.
One way to power the satellites is laser power beaming, (LPB). Laser beams can increase the power level an order of magnitude above that available from the sun. The wavelength 840 nm is within one of the transmission windows of the atmosphere, and at the same time near the peak of the photo-voltaic conversion efficiency of Si the most commonly used material for the solar panels. Beaming from the earth's surface requires the laser beam to travel through our planet's atmosphere. The atmosphere causes various problems such as scattering, absorption and distortion. The development of adaptive optics has helped solve this problems.
Free electron lasers, FELs, are capable of generating high power optical radiation without using a material medium. Unlike other lasers, which all utilize changes of electronic energy levels in a material, the light in a free electron laser is generated in a vacuum and should have no distortion. This characteristic of free electron lasers makes them ideal for generation of high power light with a diffraction limited light beam. This high quality beam can propagate through the atmosphere to great distances. This ability is due to a distortion-free initial wave front which allows all of the required corrections to result only from the atmospheric imperfections. This correction technique is now well known.
The light in an FEL is emitted from bunches of electrons traveling at very nearly the velocity of light. They are deflected by a series of magnetic poles. When the electrons are deflected, an electro-magnetic wave is radiated. The apparatus causing this deflection contains small magnets oriented somewhat like the teeth of two interlocking combs and consists of magnets with alternate north and south poles. This system is called an undulator, or in more vernacular terms a "wiggler" since it wiggles the electron bunches, which then emit light. If there is a light beam of an appropriate frequency in the vicinity of these electrons, the phenomenon of stimulated emission occurs. The electrons emit light in phase and at the same frequency as the initial light, creating Light Amplification by Stimulated Emission of Radiation or a LASER. Current state of the art for FEL generating visible light is an average power level 1-10 W. Main problems to be solved before FELs can produce hundreds of kW of optical power include (1) production of a high average current electron beam with low emittance, (2) high thermal loading in mirrors, and (3) radiation hazards from a high average power high energy electron beam.
As was mentioned above, one of the most attractive features of FELs is the possibility of generating fully transverse coherent light, having high average power. On the other hand, the efficiency of the conversion of the electron beam power to the light power is rather small in an FEL, being typically not more than a few percent. For high light power application, therefore, it is necessary to use an intense average electron beam current. In the FEL producing visible light, this beam must have high quality, i.e. it must have a low transverse and longitudinal emittance. Radio frequency (RF) photocathode guns are, in principle, capable of production of an electron beam of adequate quality, but they need a laser driver which supplies the photocathode with photons. Existing lasers generate too little average flux of photons, much less than is needed for production of an intense average electron beam current. Thus, there is an obvious problem. One can get either a high intense average electron beam current, but of poor quality, for example the electron beam current from a thermionic electron gun, or an electron beam of a good quality from the RF photocathode gun, but with low average intensity.
Another severe problem concerns the optical resonator of the FEL. Mirrors that form optical resonators become vulnerable to damage as the power level of the FEL increases.